Plain English Meaning
A vacuum pump that runs with engine oil flowing through it to keep the moving parts lubricated and to seal the chamber. Because oil passes through it, the air it discharges has to go through a separator before the oil is sent back to the engine.
Full Definition
An engine-driven vacuum pump that uses engine oil for internal lubrication and sealing. Oil is drawn into the pump, lubricates the steel vanes and rotor, helps seal the pumping chambers, and is then discharged with the exhaust air through an oil separator before being returned to the engine sump.
Context Anchor
Seen in aircraft instrument system maintenance, especially when comparing oil-lubricated vacuum pumps with dry vacuum pumps.